To test the “sulfate resistance” of a specific cement, mortar and concrete samples are generally submersed for a few weeks in concentrated Na2SO4-solutions and the mass and length gain is monitored as a function of time. High sulfate concentrations, as used in such tests, lead to the precipitation of gypsum, while lower sulfate concentrations, as present under field conditions, lead to no or only very little gypsum precipitation. The mortar samples show strongly differing expansion and mass gain depending on the reaction solution. Further factors influencing the expansion are the absence or presence of CO2 in the interacting solution, the amount of solution, the frequency of the exchange of the solutions, the sample geometry and the temperature. Considering all these factors it is not surprising that the tests carried out in different laboratories show large variations. In addition, the correlation of the test results with expansion and deterioration under field conditions is poorly understood.

The combination of thermodynamic modelling with experimental techniques such as XRD, SEM/EDS enables us to gain more insights into the process governing the interaction with the environment on a chemical level. Our studies in recent years focused on the effect of sulfate, chloride, carbonate and on alkali silica reaction.